1. Field of the Invention
The present invention is concerned with a process for the oxidative bleaching of wood pulp and for deinking waste paper with the help of hydrogen peroxide in the presence of a stabilising agent.
2. Description of Related Art
For the production of paper, besides the use of native wood pulps, to an increasing extent resource is had to recycled waste paper. The reuse of waste paper as a raw material source for the production of papers with a high degree of whiteness or brightness can only take place when, on the one hand (i), the coloured materials contained in the waste paper are substantially removed by rinsing out (in the following called deinking) and, on the other hand, (ii) the residues of coloured material are subjected to a chemical bleaching. Both procedures (i) and (ii) can be carried out in a common or in a multi-step process. The bleaching chemicals are used in order to destroy the chromophores present in the fibre materials, i.e. the coloured materials originating from the waste paper, as well as the dissolved and undissolved components of the printing colours. The bleaching can be carried out with oxidising and/or reducing chemicals. Many processes for deinking and bleaching are described in the literature. A preferred process is the simultaneous deinking and bleaching with hydrogen peroxide for waste paper and wood pulp.
The combined process is carried out, for example, with an agent of the following base composition:
______________________________________ sodium hydroxide 1.0-1.5% referred to atro material hydrogen peroxide 0.5-1.5% referred to atro material sodium silicate up to 4.0% referred to atro material complexing agent 0.1-0.4% referred to atro material soap 0.6-1.0% referred to atro material waste paper/ 0.9-2.0% referred to atro material. wood pulp ______________________________________
By "atro material" is to be understood the amount of the air-dried mixture of waste paper and wood pulp. The statements of percentage are percentages by weight.
In practice, as carrier medium there is essentially reused in a cyclic process the water originating from the process. The deinking process water has, in general, a temperature of 30.degree. to 60.degree. C. In the first step of removing the coloured material, the pH reaches a value of 9.5-10.5.
The separation of fibres and printing colours is promoted by the alkaline medium. Hydrogen peroxide has proved to be an ideal bleaching agent since it bleaches especially effectively in an alkaline medium by activation of the hydroperoxide anion (see equation 1). HO.sub.2.sup.- is the important molecule for the bleaching action. ##STR2##
During the process, as side reactions according to the following equation 2, there occurs a spontaneous dissociation of the hydrogen peroxide in the case of the presence of heavy metal ions or in the case of the presence of the enzyme catalase, as well as decomposition of the peroxide catalysed by peroxidases. They reduce the degree of utilisation of the amount of hydrogen peroxide used and thereby influence the bleaching action. The result is an increased requirement of hydrogen peroxide and aqueous sodium hydroxide solution.
Equation 2: EQU 2H.sub.2 O.sub.2 .fwdarw.2H.sub.2 O+O.sub.2
It is known to mask the damaging action of heavy metal ions by the addition of complex formers, such as ethylenediamine-tetraacetic acid, penteric acid (DTPA), polycarboxylic acids, for example citric acid, gluconic acid, polyacrylic acid, phosphonic acids and the like. These also serve simultaneously as flotation agents in the case of deinking (cf. DE 42 04 915 Al).
As natural products, waste paper and wood pulp are nutrient media for microbiological growth. Almost all micro-organisms which occur as a natural contamination on and in waste paper and wood pulp synthesise, as cell-inherent enzymes, catalases and various peroxidases.
Micro-organisms are introduced into the deinking system by the introduction of the raw materials (waste paper and wood pulp) and by the process water. The addition of hydrogen peroxide and of other chemicals, the extreme change of the pH value and the varying temperatures during the process act as stress factors on the bacteria which can lead to lysis of the bacteria. Within the process water circulation, an equilibrium is adjusted between the rate of reproduction of the micro-organisms in the system, including the bacteria introduced by the introduced material and the lysing bacteria in the system. By means of the lysis of the bacteria, the enzyme-containing cell substance, which contains catalases and peroxidases, are given off into the deinking process water.
The influence of the enzyme catalase on the bleaching activity of hydrogen peroxide in the bleaching of waste paper and wood pulp is described in the literature. Thus, G. Galland and Y. Vernac, Progr. Pap. Recycling, Vol. 2, pp. 20-30/1992, in their treatise concerning "Bleaching of recycled pulp", refer to various causes for the decomposition of peroxide during the bleaching process.
Besides traces of heavy metals, such as iron, manganese, copper and aluminium, the enzyme catalase is mentioned as the main cause of the decomposition of peroxide. Already in the case of catalase concentrations of 45 mg/liter, 60% of the hydrogen peroxide is decomposed within 10 minutes under the usual deinking bleaching conditions. The participation of catalase in the total rate of decomposition can be determined by destroying the catalase by boiling and determining the rate of decomposition of the hydrogen peroxide before and after the boiling. The difference gives the proportion of catalase.
The following methods are discussed in order to eliminate the catalase:
1. The system is to be kept free of biological activity.
2. The catalase is to be destroyed before the bleaching with:
heat treatment greater than 70.degree. C. sodium hypochlorite, concentration 0.3%
3. acid wash (see V. Gehr et al., Das Papier, pp. 186-195/1993).
These methods are not satisfactory. To keep an open technical system free from contaminations is practically impossible since, especially with waste paper, micro-organisms are continuously introduced into the system. A heat treatment costs energy and is time-consuming and, in addition, damages the paper fibres. The addition of hypochlorite is admittedly comparatively economic but also leads to a damaging of the fibres and to additional loading of the waste paper. The acid wash requires a separate process step and a subsequent expensive neutralisation, water-loading salts thereby again being formed.